Most cylinders are rated at the volume of the compressed gas, meaning 180 liters, 248 liters, 415 liters and so on. It is difficult to correlate that to the physical volume of the cylinder you plan to use without the dimensions (diameter, length, etc.)
Unfortunately, most of the European glider manufacturers only provided for 100 mm diameter steel cylinders, which as you have discovered, are difficult to find. Newer aluminum cylinders have also been in short supply lately. I was recently quoted a 52-week delivery by a normally reliable source, and it took nearly five months to get one from Mountain High. I haven't had to order any recently, so I am not sure if the supply chain has caught up.
The most significant problem with your situation is that the steel 100 mm cylinders are the only thing that will fit in the sleeve. Correspondingly sized aluminum cylinders are all 111 mm in diameter. The only other option is the KF-011 cylinder, which is a Kevlar fiber-wrapped aluminum cylinder. It is 93 mm in diameter and holds 311 liters at 1850 psi. Unfortunately, the fiber wrapped cylinders (Silica, Kevlar and Carbon) all have a finite life of 15 years, after which they are no longer certifiable. The Kevlar and Carbon fiber wrapped cylinders are lighter, but MUCH more expensive.
If it is available, the AL-180 cylinder from Mountain High may be an option. It is only 81 mm diameter and 300 mm in length. The capacity is 165 liters at 2216 psi, which is enough for 3.8 hours at 18,000 ft MSL when using the MH EDS system. I consider this to be a minimal supply and is insufficient for high desert flying or high-altitude wave flights.
Oxygen is fuel for your brain. Don't run out. It's not like running out of gasoline by the side of the highway.